The invention generally relates to a process for producing a smooth and glossy surface on a paper web, as well as a calender arrangement for carrying out such a process and, more specifically, to an improved process and calender arrangement in which the gloss effect is intensified without reducing the bulk of the web.
A similar process and calender arrangement is disclosed in German Published Patent Application 3600033. The calendar arrangement comprises two sets of rolls forming respective nips or roll gaps through which the paper web is conducted in succession. The rolls of these sets are arranged such that the hard roll of the first set of rolls presses against the paper web from one side and the hard roll of the second set of rolls presses against the web from the other side. Thus, the paper web receives the same treatment and surface formation on both sides. The hard roll, i.e., the steel roll, is heated in both sets of rolls to the minimum temperature at which the fibers of the paper web begin to deform. For paper, this temperature is approximately 175.degree. C. The high temperature produces a paper web having improved gloss without a considerable loss of specific volume, which is defined as volume per gram of paper mass and corresponds to the reciprocal value of density. A similar arrangement is disclosed in U.S. Pat. No. 4,624,744.
The calendering action that occurs when steel rolls are heated to high temperatures is referred to as temperatures gradient calendering in R. H. Crotogino's article in the "TAPPI" Journal, Vol. 65, No. 10 (Oct. 1982) at pages 97-101. As the paper web contacts a very hot roll when passing through the roll gap or nip, the fibers near the surface of the web are heated to a temperature at which they begin to become partially plasticized. As is well known, the fibers are hollow. They become permanently flattened in the partially plasticized state, which evens out the surface of the paper web. Naturally, the flattening is reinforced by the friction that occurs when the paper web is conducted against a "soft" roll coated with paper or plastic. Heat conduction from the outer layers of the paper web to the inner layers is a time dependent transfer process; in the short time that it takes for the web to pass through the nip, no considerable rise in temperature takes place in the inner layers. Thus, the temperature at the inner layers remains low such that the inner fibers do not become partially plasticized. Rather, these fibers retain their elasticity and, after passing through the gap, once again assume their former shape. Therefore, the plasticization is restricted to the outer surface layers of the web. This process generally achieves the desired effect of producing a smooth and glossy surface on the paper web, without compressing the web as a whole, by the plasticization of all its layers, to a compact mass having a low specific volume and poor quality.